RDMA/core: create struct ib_port_cache

As Jason suggested, we have 4 elements for per port arrays,
it's better to have a separate structure to represent them.

It simplifies code a bit, ~ 30 lines of code less :)

Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
Reviewed-by: Michael Wang <yun.wang@profitbricks.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>